An electrophotographic image forming apparatus may adopt a tandem system wherein a plurality of photoconductors provided in one-to-one correspondence with colors are arranged along a moving direction of a transfer medium. When forming an image, while the photoconductors are rotated, an electrostatic latent image based on successive lines of color image data is exposed on the photoconductor line by line by an exposure unit and successive lines of visible image formed on the photoconductor by developing the electrostatic latent image is transferred from the photoconductor to the transfer medium. This operation is performed in order from the upstream photoconductor to the downstream photoconductor, whereby a color image (combined image) is formed on the transfer medium.
If the rotation speed of each photoconductor is always constant, the successive lines of the electrostatic latent image is exposed on the photoconductor at a fixed time interval. Therefore, it is possible to form a color image which comprises successive lines arranged at a uniform transfer position interval. In fact, however, the rotation speed of the photoconductor fluctuates periodically and thus the transfer position intervals of the one color image may become different from that of another color image. Therefore, it may cause deterioration of the image quality and an abnormal image may be formed on the transfer medium.
If the image forming apparatus includes a transfer rotation body to which each one-line visible image is transferred directly or indirectly from the photoconductor, such as an intermediate transfer belt or a sheet conveying belt, for example, variations in the transfer position interval may also occur due to rotation speed fluctuation of the transfer rotation body as well as the photoconductor.
Then, hitherto, an image forming apparatus capable of suppressing variations in the transfer position interval caused by rotation speed fluctuation of the photoconductor has been available. (see JP-A-Hei. 7-225544) Specifically, the timing interval of exposure processing for adjacent lines is changed in response to rotation speed fluctuation of the photoconductor.
The related art image forming apparatus can suppress variations in each one-line transfer position interval. However, it is not possible to suppress deterioration of the image quality caused by the other factors.
In the related art image forming apparatus, the lengths of each of the exposure enabling time periods corresponding to each of the photoconductors are the same with each other. However, since the rotation speed fluctuation characteristic varies from one photoconductor to another, if correction processing of the exposure timing interval is performed so as to adjust the transfer position interval between successive lines of the visible image based on successive lines of image data, the number of lines of the image data to be subjected to exposure processing within the exposure enabling time period (the total number of lines) varies from one photoconductor to another and thus the quality of the image transferred to a transfer medium may be deteriorated. The transfer line interval may vary due to not only the periodical speed fluctuation of the photoconductor but also short-term speed fluctuation of the photoconductor or speed fluctuation of any other rotation body used in the exposure processing and the transfer processing, such as a conveying belt. When suppressing these fluctuations, a color image which is different in the total number of lines from one color to another may be formed.